1. Field of the Invention
The present invention relates to a method for quantitatively determining the width of a soft zone area of a partially hardened metallic workpiece having at least one hardened and one unhardened area by means of at least one multifrequency eddy current sensor.
2. Description of the Prior Art
Components that often are used in automatic transmissions are so-called planetary transmissions, whose gear teeth continuously intermesh. Despite the multiplicity of differently designed planetary transmissions, all these types of transmissions have in common that at least one gear which is designed as a planet wheel intermeshes with a centrally placed sun wheel and a peripherally running ring gear wheel. Usually so-called planet wheel bolts run through the center of the planet wheels, extending on both sides. In order to produce such type planet wheel bolts, solid or hollow cylindrical metallic rods are cut to the desired length. In order to improve the solidity of the material, the cut cylindrical pieces are subjected to a hardening process. For subsequent processing of the front ends of the individual cylindrical workpieces, the respective front ends are not subjected to the hardening process. It is known to harden the workpiece with induction hardening which effectively heats practically the entire cylindrical workpiece with the exception of the areas at the front ends of the workpiece. The front end areas of the workpiece that should not intentionally be subjected to the hardening process have, depending on the respective size of the workpiece, an axial extension, having a width, of a few millimeters, which preferably is between 1.5 mm and 2.5 mm.
The front ends of cylindrical workpieces which are partially hardened in the aforedescribed manner are then processed usually by means of a material-removing process. It is easy to understand that if the workpiece were fully hardened it would be much more difficult to carry out the finishing step and would subject the removal tool to much more wear. For this reason, there is particular interest in partially hardening, cylindrical workpieces which are half-finished products in order to ensure that the front ends of the partially hardened workpieces represent unhardened so-called “soft zone areas”.
Previously, in order to control the quality of the partially hardened half-finished products known control methods were used, such as, for example, visual inspection of the front soft zone areas which requires an educated inspection in order to be able to distinguish the hardened area of the workpiece from the unhardened one. At a suitable angle of vision and under suitable lighting conditions, light reflects in a minimally different manner. This light scattering is on the surface of the soft zone area rather than on the surface region of the hardened workpiece. This control method is expensive and time consuming. Moreover the inspection staff is prone to become tired resulting in reliability not being ensured in the desired manner.
In order to avoid employing staff, there is one known optical control measuring method known which permits detecting the color differences resulting from the light scattering of the soft zone area differing from the hardened zone.
In addition to optical methods, electro-magnetic methods are known, for example, the multifrequency eddy current method described, for example in DE 36 20 491 C2. The eddy current principle detects surface flaws as well as differences in the microstructure from irregularities in the induced eddy currents. Scanning probes or encircling probes induce these eddy currents and simultaneously measure the electro-magnetic fields generated by these eddy currents. Evaluation of the measuring signals obtained by means of the multifrequency eddy current method described in the preceding printed publication is based on elliptical evaluation in the impedance plane using a multiplicity of measuring frequencies which permits a solely qualitative finding of the to-be-examined workpiece. With the previous evaluation methods, it is not possible to state the absolute size of the soft zone areas.
Also known are so-called magnetic Barkhausen noise methods with which high-frequency Barkhausen oscillations are induced in the workpiece under examination by means of dynamic reverse magnetization processes. The high-frequency Barkhausen oscillation can be detected by a magnet-inductive receiver. The intensity of the Barkhausen noise is much more intense in the soft zone area than in the hardened zone area so that it is possible to discern and measure the differences in the two workpiece areas. A disadvantage, however, is the necessity of an excitation yoke and the great sensitivity to disturbing outside influences, which permits industrial use to only a limited extent.
DE 43 10 894 A1 describes a method and a testing probe for non-destructive examination of surfaces of electrically conductive workpieces. The disclosed testing probe should enable obtaining information about the hardness, thickness and state of the microstructure of an electrically conductive workpiece. An interaction occurs between the magnetic alternating field acting in the workpiece and the measurable voltage induced in the testing coil of the multifrequency alternating field sensor. With the aid of a multifrequency eddy current sensor, information can be gained about at least one material property of the surface under examination in the area of the penetration depth of the magnetic alternating field. In particular, the prior art method for determining the course of a hardness profile along an electrically conductive workpiece being examined serves to determine the thickness of the respective surface layers on the workpiece.